termination of nickel in high silicon steel samples (NBS SRM 1134) and a series of elementally doped glasses (NBS SRM 612-616). The results obtained are listed in Table IV. The emphasis of this study was concerned with establishing the validity of this technique, which is underscored by the agreement of the data with that supplied by NBS (21). CONCLUSIONS An exhaustive review of proton, deuteron, helium-3, and helium-4 induced activation reactions on Zn and Ni has been made. The reactions selected for sample analyses 66Zn(p,n)66Ga and 58Ni(p,pn)57Ni, respectively, are both selective and yield product nuclides with decay character(21) Provisional Certificates of Analysis, SRM's 610-616 Doped Glasses, National Bureau of Standards, Washington, D.C. (1970),
istics well suited for post-irradiation chemical separations and y-ray counting. Moreover, these charged particle activation procedures are highly sensitive. Based on experimental data, the detection limits for both Zn and Ni are estimated a t -1 ppb. This figure is based on a 3-hour irradiation with a beam current of 3 PA, using a 3- X 3-in. NaI (Tl) detector and assuming a minimum detectable peak equal to six standard deviations of the background a t the y-ray energy of interest. ACKNOWLEDGMENT The assistance of the cyclotron operations personnel is gratefully acknowledged. Received for review February 5, 1973. Accepted May 2, 1973. This work was supported by the National Science Foundation Grant GP-34877X.
Wet Chemical Oxidation Method for Carbon Determination in Ferrous Alloys William S. Updegrove' and Jon M. Baldwin2 Allied Chemical Corporation, ldaho Chemical Programs-Operations Office, ldaho Falls, ldaho 83407
A method for carbon determination in ferrous alloys was developed. Its particular utility resides in the ability to determine carbon at concentrations as low as 0.02% in a 25-mg sample, as is often necessary in the nuclear industry for nondestructive specification checking on fabricated steel assemblies. Sample dissolution and oxidation of the carbon to COS are accomplished in a closed system with a mixture of potassium peroxydisulfate, hydrochloric acid, and phosphoric acid. The COn is extracted into a stream of nitrogen and measured with a nondispersive infrared analyzer. Complete recovery of carbon was demonstrated for a wide range of alloy types. Linear working curves were obtained for 5 to 200 p g of carbon.
The determination of carbon is probably the most important and thoroughly researched analytical chemical problem in ferrous metallurgy. Although carbon is usually a minor constituent of ferrous alloys, it is that element which almost completely controls the engineering properties and gives iron its unique position as a structural material ( I ) . Recent reviews of methods for determination of carbon in steels were given in 1971 by Hines and Dulski ( 2 ) and in 1969 by Pasztor and Hines (3). Quality assurance demands of the nuclear energy industry often require a means of verifying carbon content specifications on fabricated steel assemblies. Because the structural integrity of the assemblies cannot be jeopardized for analysis, the sampling must be accomplished without impairment of function. The available sample usually consists of filings, 2
Present address. 2108 Pauline Dr.. Ann Arbor. Mich. 48104. Author t o whom correspondence should be directed.
with a total weight of less than 25 mg and a carbon concentration from 0.01 to 1.0 weight per cent. This sample size limitation prevents application of many conventional methods of carbon determination. Some methods which have the requisite sensitivity for such small samples are limited by a matrix dependent response. We describe here a wet chemical oxidation method for carbon determination in ferrous alloys that permits the use of samples suitably small for quality assurance type analyses, and which is generally matrix insensitive. The technique most frequently utilized for carbon determination in ferrous alloys is combustion of the sample and a metallic accelerator in oxygen, followed by measurement of the COz. Various methods have been used for measuring CO2, i. e . , manometry, volumetry, conductimetry, gas chromatography with thermal conductivity detection, and infrared spectrometry. Gas chromatography or infrared absorption is preferred for greater specificity. Infrared absorption has the additional advantages of greater simplicity and ease of operation in routine applications. It has been the method of choice for the latest generation of commercial instruments. Reaction gas chromatography was used in this laboratory to determine the COz combustion product ( 4 ) . The steel sample and an accelerator were combusted in an oxygen atmosphere with radiofrequency induction heating. The gaseous combustion products were separated chromatographically and the COz was catalytically reduced to CH4. A hydrogen flame ionization detector served to measure the CH4. The potential advantages of this method are high sensitivity, a 106 linear dynamic range, and extreme specificity ( 5 ) . Periodic checks of the
( 1 ) C Zapffe, "Stainless Steels," The American Society for Metals,
( 4 ) G. Webb and W. Updegrove, Allied Chemical Corp., unpublished
Cleveland, Ohio, 1949, p 87. (2) C. Hines and T. Dulski, Anal. Chem., 43, 100R (1971). (3) L. Pasztor and C . Hines, Anal. Chem., 41, 90R (1969).
work. (5) Y . Takahashi, R. Moore, and R. Joyce, Amer. Lab., 1 (7), 31 (1972). A N A L Y T I C A L CHEMISTRY. VOL. 45, NO. 12, OCTOBER 1973
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Table I. Ferrous Alloy Standards Used in this Study Chart Carbon, syrnNBS no. Alloy type % Date run bola 0.342 911 5 / 7 2 161 Ni 64-Cr 1 7 0
+
156 133a
Cr-Ni-Mo (NE 9450) Cr 13-Mo 0.3-S 0.3
0.515 0.120
11 1 b lOle
Ni-Mo (SAE 4620) Cr 1 8 - N i 9 ( S A E 3 0 9 0 5 )
0.193 0.054
166 349
Cr-Ni Ni 57-Cr 19-Co 13
0.027 0.08
v
A
A W
0 0
+ X
10/30/72 10/30/72 911 5 / 7 2 10/30/72 10/30/72 9/15/72 10/30/72 9/15/72 10/30/72
(Waspalloy)
Figures 5 and 6. 7-
D
4
sion spectrometer sources is relatively large (usually a cast ingot weighing a few tens of grams), small samples of filings cannot be accommodated by this arrangement.
EXPERIMENTAL
Figure 1. Ampoule filling operation
( a ) Removal of atmospheric C 0 2 . ( b ) filling with acid reagent, (c) final purging, ( d ) flame sealing of ampoule
reduction catalyst efficiency are required because of the possibility of catalyst poisoning. A practical lower limit on the amount of carbon that can be measured by combustion methods is imposed by the carbon content of the accelerator, sample container, and combustion oxygen. The best commercially available accelerator material of which we are aware is tin-coated copper chips which contain about 15 ppm of carbon. The carbon concentration in the accelerator can be reduced to less than 5 ppm by heating to 600-800 “C in a n oxygen a t mosphere (6). The carbon blank from all sources can be as little as 20 pg (equivalent to 0.002% C by weight in a 1-g sample) if industrial grade oxygen is used directly or 10 pg if ultrahigh-purity oxygen is used (7). Even this level is still prohibitive for many analyses because a 100-mg sample at the 0.01% C level is required just to give a carbon contribution from the sample about equal to the blank. Emission spectrometry in the vacuum ultraviolet has also been used to measure carbon in steels (8). Directreading vacuum spectrometers are commercially available and have been used for this type of analysis. Severtheless, matrix effects are generally significant, so one must apply predetermined corrections ( 8 ) or have a t hand standards approximating the gross composition and metallurgy of the sample. This is not a simple requirement if the variety of alloy types is large and not predetermined. Also, since the requisite sample for conventional vacuum emis( 6 ) E . Gibson, NASA Manned Spacecraft Center, 1 9 7 2 , personal com-
munication.
( 7 ) “Analytical Method for Carbon Analysis.” Angstrom, inc., Chicago, Ill., 1972. (8) K. Clarke, Appi. Spectrosc., 24, 229 (1970).
2116
Apparatus. T h e carbon analysis system used for this work was a commercial adaptation (Total Carbon Analyzer, Catalog No. O524A, Oceanography International. Inc.. College Station, Texas) of an instrument t h a t was originally developed to determine carbon in sea water (9, IO). The apparatus consists of several subunits. The ampoule sealing a n d purging subunit provided for flushing carbonate-derived and atmospheric CO2 from loaded sample ampoules with carbon-free oxygen (commercial grade oxygen. passed over hot CuO and ascarite) and for flame sealing the ampoules without introducing carbon from the combustion gases. A pressure vessel was used to contain the sealed ampoules during the 180 “C digestion and oxidation. The analyzing subunit was comprised of a device for opening the ampoules, a gas train for extracting the sample-derived COZ into a nitrogen stream, a magnesium perchlorate gas dryer. and a nondispersive infrared a n a lyzer for measuring the ( 2 0 2 . T h e original technique of Menzel and Vaccaro (IO) incorporated in the analyzing unit a trap containing a n acidic solution of K I to remove Cl2 liberated during the oxidation process. For this work the trap was not included a n d has been found by others to be unnecessary (11). A potentiometric recorder (Leeds and Northrup Model XL-600) was used t o display the signal for diagnostic and monitoring purposes. An electronic digital integrator with a resolution of 1 k V sec and a linear dynamic range of lo6 (Autolab Model 6300-01) was used to measure peak areas for quantitative analysis. R e a g e n t s a n d Supplies. The oxidizing agent wras analytical reagent grade solid potassium peroxydisulfate. A phosphoric/hydrochloric acid reagent prepared by mixing four volumes of a 6% v / v distilled water solution of reagent grade concentrated phosphoric acid and one volume of reagent grade concentrated hydrochloric acid was used to dissolve the sample. Inorganically derived COZ was purged from this reagent by bubbling with industrial grade oxygen which had been treated with hot (440 “ C ) CuO to oxidize hydrocarbon contaminants and passed over ascarite to remove the resulting CO2. Purging of this reagent was monitored by passing the effluent gas stream through the infrared analyzer and continued until C 0 2 evolution ceased to be observable (about 45 min a t 100 ml/min 0 2 flow for 500 ml of reagent). Subsequent contamination was prevented by continual purging at about 10 ml/min. The glass reagent dispensing flask was designed to prevent exposure of the acid mixture to the atmosphere. The small, continuous flow of oxygen prevented air from being pulled back into t h e headspace of t h e flask at the end of each reagent dispensing operation. Fresh acid mixture was prepared ,daily to minimize the chance of contamination by organic material. Weighed quantities of National Bureau of Standards ferrous alloy standards were used for calibration of the method and determination of oxidation efficiency. T h e standards are described in Table I with the symbols used to represent them on the graphs. ( 9 ) R. Wilson, Limnoi. Oceanogr.. 6, 259 (1961) (10) D. Menzel and R. Vaccaro, Limnoi. Oceanogr., 9, 138 (1964). (11) W . Sackett, Texas A & M , personal communication, 1972.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 12, OCTOBER 1973
LrnDoule T u f t e r Hand e
3,
Cutter
c
Figure 2. A m p o u l e analyzing subunit
(a) Position of cutter prior to opening ampoule, ( b ) after opening, (c) use of inorganic carbon standardization ampoule
Standard solutions of inorganic carbonate were prepared from a stock solution of 1.76488 g of reagent grade sodium carbonate in 500 ml of distilled water a t p H 7.5. Dilutions of this stock solution with distilled water ( p H 7.5) were made to give working solutions with 100,500. and 1000 p g C / m l . Ten-milliliter borosilicate glass ampoules (Oceanography International, Inc., Stock Xo. M106), without ceramic break rings, were used t o contain t h e samples during dissolution a n d oxidation. T h e ampoules were pretreated t o remove organic contaminants by flushing them with oxygen and then heating to just below t h e softening point (j50 "C) of the glass for a t least 4 hr. T h e ampoules were cooled a n d fitted with either aluminum foil or glass caps. Ampoules prepared in this manner could be stored for several weeks without picking u p carbon contamination. Analysis Procedure. A sample of filings weighing 5 to 50 mg was used for the analysis. The weighed sample was placed in a 10-ml glass ampoule with approximately 0.25 g of solid potassium peroxydisulfate. T h e ampoule was fitted with a glass purge tube t h a t delivered 100 m l / m i n of the same carbon-free oxygen t h a t was used for purging in t h e preparation of the mixed acid reagent (Figure l a ) . A glass purge cone was used to prevent back flow and mixing of ambient air. A purge time of 5 min was required to completely remove ambient air from the ampoule. The purge cone was lifted slightly t o a d m i t a 1/16-in. diameter polyethylene t u b e connected t o t h e bottle of acid reagent (Figure l b ) ; a n d approximately 5 ml of the acid was added to the ampoule. Care was taken during this operation t o disturb the purge tube and purge cone as little a s possible t o prevent entry of air into the ampoule. After adding t h e acid, t h e purge tube was raised t o about 1 m m above t h e liquid level a n d purging continued for 1 min (Figure IC). The ampoule was sealed by heating with a propane burner. The top section of t h e ampoule neck was held in a clamping device and the seal formed by lowering a n d rotating the ampoule as the glass softened. A thin top area on the seal was required for easy opening during t h e COZ measurement. T h e positions of the purge tube a n d purge cone were maintained as shown in Figure I d to prevent entry of COz and hydrocarbons from the flame into the ampoule. T h e filled and sealed ampoules were placed in metal racks and loaded into a pressure vessel. Water was added t o t h e pressure vessel to equalize the water vapor pressure inside a n d outside t h e ampoules during t h e digestion. T h e pressure vessel was sealed. p u t into a n oven, a n d maintained a t 180 "C for approximately 8 hr . Upon completion of the digestion, the pressure vessel was allowed to cool. T h e ampoules were removed a n d inserted in t h e analyzing unit (Figure 2a and b). where they were broken and purged with nitrogen to extract the COz along with other gases, which was passed through a magnesium perchlorate t r a p to remove H 2 0 and into the infrared analyzer. T h e output of the infrared analyzer was a peak of about 3-min duration. T h e area under the peak indicated t h e quantity of COz. Conversion Efficiency Measurement. Known quantities of COZ were produced from the reaction of standard Xa2C03 solutions with phosphoric acid. Five milliliters of p H 1 phosphoric acid solution were added t o a special ampoule connected to the analyzing unit a s shown in Figure 2c, a n d the acid was purged
with nitrogen until all COz was removed, as indicated by the response of the infrared analyzer. Known amounts of CO2 were generated by syringe injection of measured volumes of t h e standard carbonate solutions.
RESULTS AND DISCUSSION We discovered early in the development of this method that the operations performed prior to the digestion step were most crucial and that care taken in that part of the analysis dictated its success or failure. Anomalous results were obtained whenever adequate care was not exercised in the steps of purging, filling with acid, and sealing the ampoules. Carelessness resulted in high and erratic results due to failure to exclude completely either the ambient air or oxides of carbon and unburned hydrocarbons produced by the burner. Improper purging also leads to low results. In the first attempts a t analysis, the purge tube was allowed to remain immersed in the acid reagent during the last minute of the purging (see Analysis Procedure). Low alloy steels were readily attacked by the acid reagent and gave low results. The problem was resolved by raising the tip of the purge tube just above the surface of the acid during the . low results can be last minute of purging (Figure I C ) The rationalized on the basis of work by Abell e t al. (12, 13) and by Chang e t al. (14) who established that the action of mineral acids on carbides in lunar material and on meteoritic cohenite, (Fe,Ni)&, produces by hydrolysis a mixture of light hydrocarbon gases. It seems reasonable that the same reactions occur during dissolution of a carbide-containing steel sample and that such products are likely to be lost during the last part of the purging step if the solution is agitated by the bubbling gas. Nevertheless, certain aspects of this effect may be beneficial: e . g . . in the event the surface of the sample is contaminated with carbon-bearing substances. even adsorbed C O z . the first stages of digestion can serve as a cleaning process. An upper limit of 50 mg total sample weight was imposed by the hydrogen pressure produced in the glass ampoule during sample dissolution. About 3 atmospheres of pressure are generated in the headspace of an ampoule during dissolution of 50 mg of iron. When larger samples were used, ampoules frequently broke during digestion, Abell. P. Cadogon, G . Eglington. J. Maxwell, and C. Pillinger. Proc. 2nd Lunar Sci. Conf., Geochim. Cosmochim. Acta, Suppi. 2,
(12) P.
2, 1843 (1971). (13) P. Abell, G . Eglington, J. Maxwell, C. Pillinger, and J. Hayes, Nature (London), 226, 251 (1970). (14) S. Chang, J. Smith, I . Kaplan, J. Lawless, L. Kvenvolden, and C. Ponnamperuma. Proc. Apoiio 7 7 [Eleven] Lunar SCI. Conf.. Geochim. Cosmochim. Acta, Suppl. 1 , 2, 1857 (1970).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 12, OCTOBER 1973
2117
1248
1229
J
1141
1133
c,4
t
,
mu
I
-0
0
40
I
Integration Stop
Figure 3. Typical sample peaks Numbers next to peaks are integrals in millivolt seconds, (a) 110 pg of carbon, (b) 100 pg of carbon
50 MILLIGRAMS OF 0 5 3 5 % CARBON LOW ALLOY STEEL
T
50 mV
I
1 6 s e c i
Figure 4. Effect of sample overload on peak shape
and with those that remained intact, the pressure surge into the infrared analyzer often distorted the results. When the ampoule was broken, the COz passed through the analyzer in a peak of about 3-min duration. The peak height was dependent on several factors: the speed of lowering the purge tube into the ampoule, the pressure in the ampoule, and the relative quantities of C O z dissolved in the liquid and gas phases. Therefore, integration of the area under the peak was required for precise results, as shown in Figure 3. The peaks labeled a represent the re-
120
80 M :rogramr
Inteqration Star1
2118
% I
1
160
m
Carbon n SlondorC
Figure 5. Calibration curve for carbon in steels See Table I for interpretation of symbols
sponse of the analyzer to the COz produced from 110 pg of carbon and those labeled b represent the response for 100 pg of carbon. The difference between these two amounts of carbon was difficult to discern on the basis of peak height but was obvious from the integral of the peak. The effects of overloading the instrument, both with regard to amount of carbon and total sample weight, are shown in Figure 4. This recording is similar to those of the previous figure, with different time and voltage scales. Fifty milligrams of steel was used as the sample. This weight represented the largest sample that could be run with consistent success. The recorder tracing exhibits some of the features which prevented running heavier samples. When the pressure in the ampoule was near 3 atmospheres, various modes of COz extraction and flow become important contributors to the fine structure of the peak. Region 1 of the peak was probably caused by the portion of the COz that entered thc instrument with the initial pressure surge as the ampoule was broken. Region 2 was the bulk of the COz in the headspace of the ampoule, and region 3 was produced by the COz purged out of the liquid phase. The shapes of the three parts of the tracing changed with the speed of lowering the purge tube into the ampoule. Again, this indicated a need for integration of the output signal for reproducible results. With samples weighing more than 50 mg, the transition from region 2 to 3 becomes more gradual. The unsophisticated digital integrator used in this work was unable to accommodate overlapping peaks and would sense the end of a peak between 2 and 3, resulting in loss of part of the peak area. A more sophisticated digital integrator that could handle overlapping peaks would remove this restriction. Nevertheless, the problem of ampoule integrity with large samples would remain. The upper portion of the peak in Figure 4 was also distorted due to the large amount, 268 pg, of carbon in the sample. We have not determined if the nonlinearity is due to electronic limitations or flow properties of the analyzer. Figure 5 is a plot of total carbon in the samples us. peak integral. The stability and long-term reproducibility of
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 12, OCTOBER 1973
the system are demonstrated by the correspondence between the points designated by filled and open symbols. These sets of data were separated in time by more than a month. The data were taken on six alloy types with varying carbon concentrations. A key to the symbols is given in Table I. The least-squares fit of a straight line is also shown in Figure 5 . There is a nonlinearity of approximately 7% in the published infrared analyzer response curve (15). However, inclusion of higher degrees of the independent variable did not give a significantly better fit to the data; thus, nonlinearity was not detectable in this work. The conversion efficiency for COz production was checked by comparing the amount of carbon in steel samples with the amount recovered, as calculated by comparison with the COz released from known amounts of inorganic carbonate. The results are shown in Figure 6, where the straight line represents 100% recovery. Complete recovery, within the limits of accuracy of the method, was obtained for all alloy types except for the Waspalloy, NBS-349, which is a Ni-Cr-Co corrosion-resistant alloy. Incomplete dissolution of the Waspalloy was visually evident from the residue in the sample ampoules.
CONCLUSIONS The method presented herein provides a means of analyzing for carbon in a wide range of ferrous alloy types, with a relative accuracy of about *15% on a single sample over the range 5 to 200 pg C. The strength of the method arises from the ability to handle very small samples, permitting nondestructive sampling of fabricated assemblies. The results are insensitive to steel type, so long as the sample is completely dissolved. The method is useful for smaller sample sizes than most alternative methods. The elapsed time for a determination is relatively long-about 10 hr; however, the manpower effort per sample is roughly equivalent to that required for high-temperature combustion in oxygen if samples are run in batches. While NBS steel standards were used for calibration in the work presented here, the demonstrated complete recovery of carbon from a variety of alloy types does suggest that unless the sample is expected to be particularly diffi(15) 'LIRA Infrared Analyzer Model 300 Instruction Manual," Mine Safety Appliances Co , Pittsburgh, Pa.
200
I
I
I
I
I
I
I
I
L I60
6
120
VI
40
0
0
40
80
120
Micragromr Carbon Found
Figure 6. Recovery of CO? from steels See Table I for interpretation of symbols
cult to dissolve, a much simpler and faster calibration with inorganic carbonate is possible. Although the digestion reagent used here failed in the case of NBS-349, it was applicable to more common alloy types. Furtherinore, this does not necessarily represent a fundamental limitation of the method. More vigorous digestion conditions can be selected to accommodate exotic alloys. Analysis for carbon in nonferrous alloys should also be feasible with modified digestion conditions.
ACKNOWLEDGMENT The assistance of R. E. McAtee, D. R. Trammell, and V. R. Olsen is gratefully acknowledged. Received for review January 3, 1973. Accepted April 11, 1973. This work was performed under Contract AT(101)1375-S-72-1with the U. S. Atomic Energy Commission.
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